Ouagadougou, the capital of Burkina Faso, is currently experiencing rapid population growth, mainly concentrated in the outskirts of the city. Since 2008, the Ouagadougou Health and Demographic Surveillance System (Ouaga HDSS) has followed 80,000 people living in five neighbourhoods on the periphery of the city, half of them living in poor, informal settlements. This urban population faces a complex burden of disease: children under five die mainly from infectious diseases, while the main causes of deaths among adults aged 15–59 are AIDS, cardiovascular diseases and accidents. In this analysis, we explore whether poverty is associated with greater mortality among small children and among adults and we examine how the risk factors associated with the main causes of death in these two age groups vary by socio-economic status. We find that children who are born to uneducated and poor parents are twice as likely to die as their counterparts likely because they have worse access to both preventive and curative health care. Young children living in informal areas are also twice as likely to die compared to others because, everything else being equal, they are more often ill, more often malnourished, and less likely to receive medical care; these outcomes can be related to the unsanitary environment. In contrast, adult mortality is higher in formal neighbourhoods. Since adult health behaviours are similar regardless of neighbourhood (everything else being constant), this excess mortality seems attributable to the relatively higher wealth of households in formal neighbourhoods: affluent adults are more often overweight and more likely to be HIV positive. Better educated adults have a lower risk of dying than others because, despite their higher rates of accidents and HIV infection, they are more likely to seek medical care.